1. Field of the Invention
The present invention relates to a method for producing a film packed battery whereby a battery power source comprising a cathode, an anode, an electrolyte and a separator are packed with a heat sealable film, and in detail, a method for producing a film packed battery which is prevented from deformation during charge and discharge processes with excellent stability.
2. Description of the Prior Art
With increasing the number of the notebook computer and mobile phone, there is an increased demand for developing a battery with high power and excellent stability. In order to meet demand, a secondary battery utilizing electrochemical reaction of alkaline metal ions such as lithium ions has been developed. This type of a secondary battery is comprised in that an anode active material layer and cathode active material layer are separated by a separator made of a porous film and wound or stacked, and then enclosed in a metal can case.
A prismatic battery which can be efficiently mounted on various electronic equipment has been also developed. Such prismatic battery is formed in that the electrodes-separator stacked layers are prismatically wound, pressed by using flat plates, and packed in a metal case. However, recently, there are also demands for electronic equipment including its package materials to be miniaturized and made lightweight, and to make its package thinner and lighter is demanded However, a battery packed with the metal case is heavy because the case is required to be thick and stiff, and it is barely made thinner than 4 mm in terms of case formation, and therefore, to meet such demands was difficult.
Therefore, a package comprising a heat sealable film has been examined. This film is made by disposing, on the exterior surface of metallic foil such as aluminum, a resin such as nylon and polyester having high mechanical strength and, on the interior surface, a resin such as polyethylene and polypropylene having a heat sealable property and an excellent chemical stability. Further, inside the package, a battery comprising cathode active material layer and anode active material layer which are separated by a separator made of a porous film and wound or stacked is placed, the interior surfaces are put together and heat sealed, and thus a battery is produced.
However, there are such problems in that, the battery packed with the heat sealable film cannot suppress deformation of the battery power source by itself and thus, even when the battery power source is slightly expanded or the internal pressure is slightly increased, deformation occurs. In general, it is important for battery properties such as energy density, capacity, current density, impedance, and cycle life that the cathode active material layer and anode active material layer are in close and uniform contact via the separator. Therefore, when expansion or deformation of the battery power source occurs, impedance increases, whereby discharge capacity decreases, and cycle characteristics deteriorate.
With respect to the problems, for example, in Japanese Laid-open Patent Publication No. 5-182649 of 1993, a method for maintaining airtightness of a battery power source comprising an anode, a cathode, an electrolyte, and suppressing deformation by maintaining the inside of a heat sealable film at a pressure lower than atmospheric pressure is disclosed. Nonetheless, even when the inside of the battery is maintained at the pressure lower than atmospheric pressure, since the force for pressing the battery power source is 1 atm. at most, it is insufficient for prevention of deformation and also, when the internal pressure is increased due to generation of a gas, no effect is provided.
The object of the present invention is to provide a simple and easy method for producing a film packed battery, which is excellent in stability, that is, deformation due to charge and discharge is small, and it is unlikely that deformation and breakage of the battery occurs even when the internal pressure increases, and in particular, a simple and easy method suitable for producing a thin and lightweight film packed battery.
Another object of the present invention is to provide a manufacturing method of a prismatic battery that reduces quality variations even when pressured charging is simultaneously performed on a plurality of batteries by use of one pressing apparatus having a simple structure
The first aspect of the present invention relates to a method for producing a film packed battery whereby a battery power source comprising, at least, a cathode, an anode, and a separator are packed with a heat sealable film, wherein a pre-charging step for charging while pressure is applied is included at least once.
The second aspect of the present invention relates to a method of manufacturing a prismatic battery, comprising at least one pre-charging step in which at least one prismatic battery is sandwiched between a pair of pressing plates having a buffer layer at least on one of the pressing plates, and under a condition where the interval between the pressing plates is fixed at a predetermined interval that enables pressing of the prismatic battery, the prismatic battery is charged while pressure is being applied to the prismatic battery.
As the buffer layer, for example, a rubber resilient member in a sheet form is used. Furthermore, the buffer layer may comprise at least two layers of a high hardness layer and a low hardness layer. In this case, the high hardness layer is disposed so as to be in contact with the prismatic battery. At this time, it is preferable that the low hardness layer be a rubber resilient member in a sheet form, and it is preferable that the high hardness layer be made of a metal sheet or a resin sheet being harder than the low hardness layer.
Moreover, the present Invention is particularly advantageous when a plurality of prismatic batteries is sandwiched between the pair of pressing plates in the pre-charging step to simultaneously manufacture a plurality of prismatic batteries.
According to examination by the inventors, it was proved that after packing the battery power source with the heat sealable film, by charging at least once while pressure is applied, deformation due to charge and discharge in use thereafter is small. It is understood that, for the film packed battery produced by the method according to the invention, the internal strains due to charge are averaged over the whole battery, and deformation is also suppressed when charging and discharging thereafter Therefore, possibilities such as detachment between the collector and electrode layer or between electrode layer and separator and breakage of the separator caused by deformation of the battery power source, and an increase in impedance, decrease in capacity or charge and discharge efficiency, and internal short circuit resultant therefrom can be reduced.
As has been described, according to the present invention, the film packed battery, which is excellent in stability and safety, that is, there is little possibility of deformation and breakage of the battery even when the internal pressure is increased through use, can be simply and easily produced. The production method according to the present invention is extremely efficient especially when a thin and lightweight battery is produced.
According to the present invention, a film packed battery in which the possibility of deformation and breakage of the battery is small even when the internal pressure is increased through use, deterioration of various characteristics, in particular, deterioration of cycle characteristics is small, and which is excellent in stability and safety can be easily produced even when the internal pressure is increased through use.